<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<style>
  body>div{
    width: 200px;
    border: 1px solid red;
    /*当元素的所有子元素全部浮动时 自动识别的高度为0后面的*/
    /*元素会顶上来导致显示异常，添加overflow：hidden解决此异常*/
    overflow: hidden;
  }

  #d1{
    width: 50px;
    height: 50px;
    background-color: red;
    float:left;
  }

  #d2{
    width: 50px;
    height: 50px;
    background-color: green;
    float: left;
  }

  #d3{
    width: 50px;
    height: 50px;
    background-color: blue;
    float: left;
  }

</style>
<body>
<div>
  <div id="d1"></div>
  <div id="d2"></div>
  <div id="d3"></div>
</div>

</body>
</html>